A hybrid vehicle equipped with an engine and a motor-generator has been developed to reduce emission and improve fuel economy. In the hybrid vehicle, an exhaust heat of the engine is utilized for heating a passenger compartment or warming up the engine. It is known that an electric power generation of the engine is increased when a heating requirement is generated. For example, Japanese patent No. 5206495 shows that a charge and discharge amount of the battery is controlled to decrease a state of charge (SOC) of the battery when a heating requirement is generated.
In the above conventional art, when a heating requirement is generated, the engine is restarted without respect to the SOC of a main battery so as to ensure the heating energy. Furthermore, in order to ensure the heating energy more, the engine needs to generate more output energy corresponding to a load of the main battery in addition to a travelling load. Thus, when the heating requirement is generated in a state where the SOC is relatively high, the SOC reaches an upper limit immediately and the main battery cannot be charged anymore. Then, the engine is driven in a low load in order to ensure the heating energy only. The engine efficiency is deteriorated. Also, since the engine output is small, the engine operating time is prolonged.